Hard to tell. but it's not 40"
Some videos it looks only 2 feet.That one photo looks like 3 feet maybe

I'm not sure how to imbed images on here...but there's pic on Google images of chamberlain at rim-level with his head on an in-game dunk....for him to accomplish that:

height = 85"
rim = 120 inches......he'd have had to jump at least 33-35" to dunk it like that....now you don't believe that he could do more if there was no ball, and he actually prepped for the jump??